1. Field of the Invention
This invention relates to vacuum pumps and more particularly to an ion pump for producing an ultrahigh degree of vacuum.
2. Description of the Prior Art
An ion pump has heretofore been used in general for the purpose of exhausting air or other gas from an enclosed space for experimental apparatus, manufacturing apparatus in electronic industry or the like to a high degree of vacuum of at least 10.sup.-6 Torr without polluting the space with oil vapor or the like. In the ion pump, moving electrons in a high electric field collide with gas molecules in the high electric field to ionize the gas molecules to produce ions. The ions thus produced collide with the electrode formed of titanium or the like and having a getter action and one portion of the ions is captured by the electrode. In this case, the energy produced during the collision of the ions with the titanium electrode causes the atom of titanium or the like to spatter and the atom spattered is continuously adhered to the other electrode having a large surface area. The above mentioned capture of one portion of the ions produced and adsorption of gas molecules due to the spattering of the titanium atom or the like function to exhaust air or other gas from the enclosed space to a desired degree of vacuum.
A conventional ion pump has the drawback that as the degree of vacuum in the enclosed space becomes high the exhaust speed becomes extremely low. If the exhaust speed becomes low, it takes not only a long time to exhaust air or other gas from the enclosed space to a desired degree of vacuum but also a vital drawback that a degree of vacuum to be obtained in an exhaust system as a whole becomes low. The cause of extremely lowering the exhaust speed is as follows. If the degree of vacuum becomes high, the means free path of the moving electrons becomes long to decrease the probability of ionizing the gas molecules per one electron, and as a result, the number of electrons contributing to the successive ionization is decreased. This is due to the fact that there occurs a negative feedback action in the course of producing ions.
In the conventional ion pump, in order to improve the ionization probability per one electron, a magnetic field is applied to a space in which the electrons are moving so as to cause the electrons to effect their rotary motion and make the travelling distance of the electron long. But, it has been impossible to eliminate the above mentioned negative feedback action.